Hi, I've never seen this before as I am a new duck owner. Does your duck have any other signs or symptoms of illness?
There is a picture in my hobby farms Duck magazine of a full-size adult duck with what looks like a large, irritated blister covering half of its eye. The description says "A
prolapsed eyelid may be a sign of botulism." Like I said, in that case the lump is large and irritated covering about half of the eye, unlike the bump on your ducklings eye, but maybe it is something to consider? You can find images of prolapsed eyelids on google.
I googled a bit more and found another disease with similar symptoms--
avian pox--maybe also worth looking into? This page explains pox and other common eye disorders in birds:
http://www.petmd.com/bird/conditions/eyes/c_bd_Eye_Disorders
It could be an infection of the affected area. Do you have any "head washers" for the duckling? Just a bowl or cup of water that he/she can fully submerge his/her head in? It helps them to keep their eyes, nares, and mouths clean to avoid infection. But since this duckling looks so little, I would make sure to supervise him while he is cleaning, and then take away the head washer after a few minutes when he seems like he's had enough head dunks. Maybe try this a couple times a day?
I hope other people will have more specific answers for you... and I hope it turns out to be nothing too serious!